1. Field of the Invention
This invention relates to a wheel speed sensor for a drive axle and, more specifically, to such a sensor which can be located inwardly of the wheel, for example, in the differential area and may be utilized in an anti-wheel lock brake control system. U.S. Pat. Nos. 3,743,362 and 3,790,227 are hereby incorporated by reference.
2. Description of the Prior Art
Modern anti-wheel lock brake control systems operate in response to electrical signals indicative of the angular velocity of one or more of the vehicle wheels. Usually these signals are derived from sensors which include an electromagnetic sensing device having at least one magnet and carried by a stationary portion of the vehicle such as the axle housing and a toothed or notched metal rotor which rotates in response to wheel rotation and is located opposite the sensing device. As the rotor teeth pass the electromagnetic sensing device, the resulting variations in flux produce a voltage, the frequency of which is a function of the angular velocity of the wheel and the number of teeth in the rotor.
To produce the required voltage level of the velocity signal, the sensing device and excitor rotor must be accurately located with respect to each other during initial assembly and must remain in this preselected relationship even after extended periods of operation in which the components are subjected to road shocks and repeated vibration. It is also essential that installation of the sensor assembly be accomplished in a manner adaptable to mass production methods to minimize the need for skilled technicians and elaborate installation techniques. It has heretofore been the general practice to provide wheel speed sensors for braking systems which are located in the wheel region. It has, however, been found that these sensors have caused some problems by their location at the wheel since care must be taken to prevent their damage during wheel maintenance.
Although, as shown in U.S. Pat. Nos. 3,138,970 and 3,769,533, consideration has been given to wheel speed sensors for drive axles which are located in the differential area of the axle, their acceptability has been limited. In these systems, a rotor is directly mounted to the axle shaft and the electromagnetic sensing device is mounted on the differential housing for extension therein in alignment with the rotor.
However, with improved anti-wheel lock brake control systems, better braking control and shorter stopping distances have been obtained when a higher number of teeth are used in the rotor, for example, 60 to 120 teeth. Increasing the number of teeth for better resolution of the system also requires that the distance between the sensing device and the rotor be minimized and places greater emphasis on maintaining this distance within predetermined limits throughout rotor rotation. This has been found to be significant with an anti-wheel lock brake system such as disclosed in U.S. Pat. Nos. 3,743,362 and 3,790,227, which are incorporated by reference herein. The system disclosed therein, for example, is an axle-by-axle system which includes for each axle a digital computer for controlling a high capacity valve which is capable of delivering maximum operating air pressure to both brakes on the axle. The computer logic automatically controls the valve so that it is capable of delivering only a proportion of the operating air pressure in an effort to keep the level of air pressure just below that at which either wheel will lock. The computer must rely on accurate wheel speed indication to be capable of making constant decisions at a rate of about 50 times a second on whether to increase or decrease air pressure, or maintain it at the same level. Because the system is capable of proportioning the air pressure rather than being limited to simply turning it fully on or off as was the practice in many systems heretofore utilized, it produces a smoother, controlled stop in shorter distances.
Mounting a rotor directly to an axle shaft, as taught by the prior art discussed hereinabove, has generally been found for several independent reasons to be incapable of satisfying the requirements of these improved systems. Firstly, the normal manufacturing tolerances for an axle shaft and the designed "float" allowed for the side gears of the differential will not insure a sufficiently accurate rotor location with respect to the portion of the drive axle housing on which the sensing device has been mounted. Without redesigning to eliminate the "float" and/or without significantly decreasing the tolerances of the axle shaft during its manufacture, the portion of the shaft to which the rotor is secured can be inaccurately aligned with the housing so that the axle shaft will move transversely within the housing during its rotation.
Additionally, since during initial installation the axle shaft must be physically inserted into the differential through the interior of the drive axle housing, potential damage to the rotor mounted thereon is possible by its accidentally making contact with the interior. This potential damage to the rotor during installation also exists each time the axle shaft must be removed for maintenance purposes.
Further, since the end of the housing adjacent the wheel area has an inside diameter only slightly larger than the outside diameter of the axle shaft, the size of the rotor is also limited if it is permanently and directly mounted thereon thus limiting the number of teeth that can be utilized for improving system resolution.
Still further, the fact that the axle shaft is sometimes removed for maintenance presents another reason, perhaps as significant as any presented hereinabove, for not having the rotor permanently mounted on the axle shaft. When the rotor is no longer protected by the housing and is generally exposed to the environment of a maintenance area, damage to the surface or teeth of the rotor is a significant possibility which could seriously affect the operation of the wheel speed sensor when it is reinstalled.